Gold plating bath for barrel plating operations

ABSTRACT

AN ELECTROPLATING BATH PARTICULARLY SUITED FOR BARREL PLATING OPERATIONS. THE BATH INCLUDES IN ADDITION TO AN ALKALI-GOLD CYANIDE, A BATH SOLUBLE SA&#39;&#39;T OF N,N-DIH-2-HYDROXYETHYLGLYCINE OR OF GLYCINE, OR GLYCINE ITSELF MAY BE UTILIZED. THE BATH FURTHER INCLUDES QUANTITIES OF HYDRAZINE, SMALL AMOUNTS OF ARSENIC AND/OR LEAD ION, AND A BUFFERING AGENT SUCH AS PHOSPHATE ION.

United States Patent O Int. Cl. C23b /28 US. Cl. 204-46 14 ClaimsABSTRACT OF THE DISCLOSURE An electroplating bath particularly suitedfor barrel plating operations. The bath includes in addition to analkali-gold cyanide, a bath soluble salt of N,N-di-2-hydroxyethylglycineor of glycine, or glycine itself may be utilized. The bath furtherincludes quantities of hydrazine, small amounts of arsenic and/or leadion, and a buffering agent such as phosphate ion.

This application is a continuation-in-part of our copendingapplications: Ser. No. 273,860, filed July 21, 1972, for Gold PlatingBath for Barrel Plating Operations; and Ser. No. 288,020, filed Sept.11, 1972, for Gold Plating Bath for Barrel Plating Operations. Each ofthe said applications is assigned to the same assignee as the instantapplication.

BACKGROUND OF INVENTION This invention relates generally toelectroplating baths, and more specifically relates to such baths as areuseful in barrel gold plating operations.

Gold, within recent years, has become a very important part of theelectronics industry. Among those properties recommending its use areits relative inalterability, high solderability, and low contactresistance. In the semiconductor field, gold has furthermore found favorbecause of its ability to readily form a eutectic alloy with silicon andgermanium.

In the latter connection, it may be noted that most headers or packagesfor diodes, transistors, and integrated circuits are gold plated as apreparation for the mounting or attaching of the semi-conductor devices.For such an application, the gold deposit must be of high purity anddeposited as uniformly as possible, in order to readily alloy withsilicon or other metallic contacts. The problem of plating such parts iscompounded by the fact that these components are irregularly shaped andof complicated design. Such parts are exemplified by the well-known lineof TO-5 and TO-8 multi-lead headers. Such headers consist of an eyeletof Kovar metal to which several insulated Kovar leads are attached andsealed in glass.

In accordance with known principles in the art, headers of the foregoingtype have in the past been plated (among other methods) by so-calledbarrel plating techniquesthat is, by subjecting such articles toelectroplating while a plurality of articles tumble in a barrel. Whensuch articles are thus plated, however, it is found that many leads donot make electrical contact with the remainder of the load. Where suchcondition obtains during the plating cycle, the portion of the leadclosest to the anode becomes cathodic. Such leads become bipolar, and atthe anodic portions of the leads problems can arise in that (a) the goldmay redissolve anodically; and (b) the base metal can be attached toexpose bare spots. Should the tumbling action be markedly inadequate,these problems can become quite severe. In the past these problems havepartially been overcome by incorporating mechanical means 3,791,941Patented Feb. 12., 1974 for improving the electrical conductivitythrough the load. Such means have taken the form of metal particles ormetal shot. Unfortunately, during the plating operation, the shot itselfbecomes gold-plated, resulting in loss of gold and attendant increase inthe cost of plating the desired objects-i.e. the headers, etc.

In our above-cited co-pending application Ser. No. 273,860, we havedisclosed our finding that an alkali-gold cyanide electroplating bathincluding a chelating phosphonic acid, with additional quantities ofhydrazine and small amounts of arsenic and/ or lead ion, eliminates orminimizes many of the cited problems, and in fact provides superiorperformance in the aforementioned barrel plating operations. The saidbaths, however, are in at least one important commercial respect lessthan satisfactory: we refer to the fact that the phosphonic acidcompounds identified therein are relatively expensive, which tends toinhibit the otherwise advantageous use thereof.

In the above connection it is pointed out that it is known (see forexample US. Pat. No. 3,551,305) that certain salts of the chelatingamino acids, such as, for example, salts ofN,N-di-2-hydroxyethyl-glycine are useful chelating agents for goldplating operations. These compounds, furthermore, in comparisonparticularly to the phosphonic acid compounds mentioned above, arerelatively inexpensive. Unfortunately, however, the use of thesecompounds has, in the past, been sharply limited by the fact that theylack stability under anodic oxidation. In consequence of this,decomposition products have tended in the past to form in gold platingbaths in which such materials are employed, with resultant contaminationof the bath, or of the deposit. In order to rectify such condition ithas, in the past, been required to regularly remove the decompositionproducts, as, for example, by means of activating charcoal, or the like.This phenomenon in turn has compounded the operating procedures andproduced attendant cost and loss of operating time.

In accordance with the foregoing it may be regarded as an object of thepresent invention to provide electroplating bath compositions for use inbarrel plating operations, which are highly effective in reducingeffects of bipolarity.

It is a further object of the invention, to provide gold electroplatingbaths, for use in the barrel plating of electronic components or thelike, which baths display reduced tendencies to attack the base metal ofthe said components.

It is another object of the invention, to provide gold electroplatingbaths, especially suited for use in barrel plating of electroniccomponents or the like, which prevent or slow down co-deposition withthe gold of the troublesome common inpurities such as copper, nickel,cobalt, iron and lead.

It is a further object of the invention to provide gold electroplatingbaths enabling improved metal distribution and superior aestheticproperties in the resultant platings.

It is a still further object of the invention to provide goldelectroplating baths which incorporate relatively inexpensive chelatingagents, and which incorporate addi tional agents which act to preventthe formation of undesirable decomposition of products in consequence ofanodic oxidation.

SUMMARY OF INVENTION Now in accordance with the present invention, ithas unexpectedly been discovered that the foregoing and other objectsare achieved in an electroplating bath which ineludes in addition to analkali-gold cyanide, a bath soluble salt of N,N-di-Z-hydroxyethylglycineor of glycine together with quantities of hydrazine and small amounts ofarsenic and/or lead ion. Glycine itself (amino-acetic acid) may also beused instead of or in addition to the said salts. Phosphate ion ispreferably present as a buffer; alternatively or in addition, otherbuffering agents may be utilized. The said combination in addition toeliminating or minimizing many of the cited problems previously found toexist during barrel plating operations, also (by virtue of its use ofthe cited chelating agents) enables formulations at relatively low cost.The cited chelating agents appear to function in the present environmentto reduce the tendency of the compositions to attack the base metal, andby virtue of their chelating characteristics slow down or preventco-deposition with the gold of the common impurities, such as copper,cobalt, nickel, iron and lead.

The electrolyte is kept slightly on the reducing side by the addition ofthe reducing agent, hydrazine. Such agent appears in the combination ofthe invention to greatly reduce or even eliminate the aforementionedtendency to anodic deplating of the parts being processed. Furthermore,for reasons that are not completely understood, the hydrazine, in thepresent composition acts to improve the throwing power of the bath.Additionally, a synergistic effect appears to occur in the present bathbetween the chelating agent and hydrazine, which acts to stabilize thepresent bath, well beyond that which might be anticipated on the basisof the individual components. One of the consequences of suchsynergistic effect appears to be a marked reduction in the tendency ofthe chelating agents to anodically oxidize. As a result decompositionproducts are maintained at minimal levels.

The trivalent arsenic ion and/or lead ion serve in the presentenvironment to augment the smoothness of the deposit, yielding platingsof superior aesthetic qualities, such as with respect to lustre andcolor thereof. The addition of as little as 1.5 mg. of As+++ or 2 mg. ofPb++ per liter of solution, refines the grain of the deposit andprovides an attractive pale yellow semi-bright gold deposit ofattractive lemon-yellow color and of low porosity. For reasons that arenot completely understood these additives appear in the environment ofbarrel plating operation, to greatly improve the resulting metaldistribution.

DESCRIPTION OF PREFERRED EMBODIMENT Among the chelating agents which maybe utilized in the baths of the present invention are the bath solublesalts of N,N-di-Z-hydroxyethylglycine (DHG, and of glycine (amino-aceticacid), as well as glycine itself. A preferred compound of this type foruse in the invention, is sodium dihydroxyethyl glycinate, available fromGeigy Industrial Chemicals under the trade name Chel 185. A similarcompound is also available from the Cowles Chemical Division of StaufferChemicals under the tradename Chelon DHG. The cited chelating agents arepresent in the baths of the present invention in a preferredconcentration range of from 50 to 250 .g./l. The compounds cited exhibithigh stability constants, in particular exhibiting relatively high pKvalues with respect to chelated metallic impurities, such as forexample, copper, nickel, cobalt, iron, etc.

EXAMPLES 1-4 A series of four gold electroplating baths were prepared inaccordance with the present invention, incorporating the followingcomponents (where DHG refers to sodium dihydroxyethyl glycinate):

Bath 1 2 3 4 All of the baths cited above were operated at 50 C. and atcurrent densities of 5 a.s.f., and were found to be highly suitable forbarrel plating of parts such as the 4 cited TO-S headers. Baths 2, 3 and4 were found to be relatively unstable on standing; baths 3 and 4 brokedown after three (3) days. Bath 2 was of superior stability, breakingdown after a week.

Example V A preferred bath in accordance with the invention, wasprepared as follows:

pH adjusted by small quantities of KOH or NaOH Very satisfactory golddeposits were obtained from this bath, which was found to be very stablein production. In this example the potassium phosphate monobasic andboric acid serve as buffers. In general, it is preferable with the bathsin the invention to adjust the pH with such buffers to about 7.0.Phosphoric acid may also be utilized for this purpose, as may otherbuffering systems based on borates or citrates. It may be noted that inthe absence of buffers of these or similar types, relatively inferiordeposits are obtained. This is likely due to the fact that the chelatingagents specified are amino acids of the type known to form internalsalts. The amino and carboxyl radicals of such acids saturate themselvesintramolecularly just as an amine is neutralized externally by acarboxylic acid. Since DHG contains only one basic (NH group and onecarboxyl group, it reacts almost neutral. It appears therefore since theaddition of acids are required to yield superior results, that some freeor unneutralized acids must be present in the solution.

The As+++ ion is preferably present in minimum concentration of 1.5 to 2mg./l. calculated as As+++. In place of, or in addition to the As+++ion, it has been found that small quantities of Pb++ ion may beutilized, with consequent marked improvement in the properties of thedeposit-including refinement of grain, production of porosity, andbetter metal distribution. Resultant improvement of aesthetic qualities,such as lustre and color is similarly evidenced. Where lead is usedalone as little as 2 mg./l. are thus effective (calculated as themetal), with the useful range of addition extending up to about 12mg./l. The lead may be in the form of bath soluble salts, such as leadnitrate and lead acetate. It may be observed in connection with thepresent results that it is quite unexpected in view of the fact thatlead ion acts as a most undesirable impurity in pure gold depositsplated from citrate or phosphate-citrate systems not including the othercomponents of the present baths.

Example VI A bath in accordance with the invention, was prepared asfollows:

Glycine (amino-acetic acid) g./l Hydrazine 64% ml./l 20 Phosphoric acidto pH to 7.0.

As+++ mg./l 2 Au g./l 8

at 500 C. for 5 minutes. The resultant plating was of good distributionand no discoloration developed.

The gold cited for use in all of the foregoing examples is calculated asthe metal and is added to the compositions preferably as a standard 68%P.G.C. (potassium gold cyanide) solution. Other alkali-gold cyanidesolutions, such as sodium, or ammonium gold cyanide solutions may beused. As metallic gold, the element may be present in the approximaterange of from about 2 g./l. to saturation, with a practical upper limitbeing about 32 g./l. The hydrazine 64% (hydrazine hydrate) is a productavailable from Fairmount Chemical Co., Newark, NJ. The said hydrazine64% may be present in the bath in the approximate range of from 5 ml./l.to 100 ml./l., or somewhat higher. In preparing the bath, the severalcomponents are added to the bath in the order indicated, with suflicientwater being added to provide one liter of solution. The preferredconcentration of hydrazine (calculated as 64% hydrazine) is about -20ml./l. for typical applications, such as where headers like the TO-S andTO-8 are plated. Concentrations substantially beyond this level yieldlittle additional benefits.

The baths of the invention are typically used at a pH of about 7.0, witha preferred operating range being from about 6.0 to 8.0. Typicaloperating temperatures are from about 32 C. to 54 C., with a preferredoperating range being from about 49 C. to 52 C. Maximum increasedstability of baths in accordance with the invention are achieved in thepH range of 6.0 to 8.0. It will be appre+ ciated by those skilled in theart that such result is unexpected as the reducing abilities ofhydrazine would normally be anticipated to increase with increasing pH,especially in the alkaline range between 7.0 and 8.0.

While the present invention has been particularly set forth in terms ofspecific embodiments thereof, it will be understood in view of theinstant disclosure, that numerous variations upon the invention are nowenabled to those skilled in the art. The compositions set forth, forexample, while being particularly adapted for use in barrel platingoperations, also find application in other plating environments, as forexample in rack or still plating operations, where it is chiefly thedistribution of metal which is of concern. Accordingly the invention isto be broadly construed, and limited only by the scope and spirit of theclaims now appended hereto.

We claim:

1. An electroplating bath for barrel plating of gold, comprising anaqueous solution of an alkali gold cyanide, said gold being present inconcentrations of from about 2 g./l. to saturation calculated as themetal; for about 50 to 250 g./l. of a chelating agent selected from thegroup consisting of glycine and the bath soluble alkali metal salts ofglycine and N,N-di-Z-hydroxyethylglycine; from about 5 ml./l. to 100mL/l. of hydrazine calculated as 64% hydrazine; and as an agent forimproving the distribution and aesthetic qualities of the deposit, oneor more agents selected from the group consisting of arsenic and leadion, said ion being in suliicient quantity to provide a smoother andmore evenly distributed deposit than is obtained in the absence thereof.

2. A composition according to claim 1, further including quantities ofbuffers, adequate to adjust the pH of said bath between 6.0 and 8.0.

3. A composition according to claim 2, wherein said ion agent is presentin concentrations of less than about 12 mg./l.

4. A composition according to claim 3, wherein said ion agent comprisesAs+++ in a concentration range of from about 1.5 to 12 mg/l.

5. A composition according to claim 3, wherein said ion comprises Pbin aconcentration range of from about 2 to 12 mg./l.

6. A composition according to claim 2, wherein said chelating agentcomprises the sodium salt of N,N,-di-2- hydroxyethylglycine.

7. A composition according to claim 2, wherein said chelating agentcomprises sodium glycinate.

8. A composition according to claim 2, wherein said chelating agentcomprises glycine.

9. A composition according to claim 2, wherein said buffers includephosphate ion. 1

10. A method for improved electroplating of complexshaped metallicarticles comprising: subjecting said article to electroplating in abarrel-type operation utilizing as an electroplating bath an aqueoussolution of an alkali gold cyanide, said gold being in concentrations offrom about 2 g./l. to saturation calculated as the metal; from about 50to 250 g./l. of a dissolved chelating agent selected from the groupconsisting of glycine and the bath soluble alkali metal salts of glycineand N,N-di-2-hydroxethylglycine; as an agent for reducing bipolarityeffects, from about 5 mL/l. to mL/I. of hydrazine calculated as 64%hydrazine; and as an agent for improving the distribution and aestheticqualities of the deposit, less than 12 mg./l. of an ionic agent selectedfrom the group consisting of arsenic and lead ion, said ion being insufiicient quantity to provide a smoother and more evenly distributeddeposit than is obtained in the absence thereof.

11. A method according to claim 5, wherein the pH of said bath ismaintained between about 6.0 and 8.0.

12. A method in accordance with claim 11, wherein said plating operationis conducted at temperatures of from about 49 to 52 C.

13. A method in accordance with claim 11, wherein said ion agentcomprises As+++ in the concentration range of from about 1.5 to 12mg./l.

14. A method inlaccordance with claim 11, wherein said ion agentcomprises Pb++ in a concentration range of from about 2 to 12 mg./l.

References Cited UNITED STATES PATENTS

